1. Field of the Invention
The present invention relates to a photoelectric sensor and, more particularly, to a photoelectric sensor of a separation type including a light projecting and receiving unit optically coupled to a controller through a light propagation member such as an optical fiber.
2. Description of the Related Art
JP-UM-A-1-77234 (Patent Literature 1) discloses a photoelectric sensor of a reflection type that can be confirmed an operating state with a distal end portion of a light propagation member, that is, an optical fiber. The photoelectric sensor is a photoelectric sensor of a separation type including a light projecting element for detection (an LED) optically coupled to an end of an optical fiber for light projection and a light receiving element (a photodiode) optically coupled to an end of an optical fiber for light reception. A plurality of optical fibers for operation display are disposed around the optical fiber for light projection. The photoelectric sensor further includes a first plurality of LEDs for operation display and a first plurality of LEDs for stable operation display optically coupled to ends of the plurality of optical fibers for operation display. Similarly, the photoelectric sensor further includes, around the optical fiber for light reception, a second plurality of LEDs for operation display and a second plurality of LEDs for stable operation display optically coupled to ends of the plurality of optical fibers for operation display.
During normal operation, detection light from the light projecting element (the LED) is projected by the optical fiber for light projection. When the light is reflected by an object, the reflected light is detected by the light receiving element through the optical fiber for light reception.
When the light reception is stable, that is, in a state in which the object is stably detected by a comparator circuit having a threshold level higher than a threshold level, the first and second LEDs for stable operation display are lit. This state can be confirmed by the distal ends of the optical fibers. When the object is detected, the first and second LEDs for operation display are lit. The detection can be confirmed by the distal ends of the optical fibers.
JP-A-6-85644 (Patent Literature 2) proposes a photoelectric sensor having a purpose of simplifying positioning of an optical system, that is, relative positioning of an optical fiber for light projection and an optical fiber for light reception, that is, optical axis alignment in setting of the photoelectric sensor in a production line based on the premise that a main body, that is, a controller of a separation-type photoelectric sensor includes an operation display lamp and a stable operation display lamp.
In the photoelectric sensor proposed by Patent Literature 2, an LED for light projection (red) and an LED for display (green) are disposed side by side. The LEDs are optically coupled to a common optical fiber for light projection (detection optical system). During detecting operation, both of the LED for light projection (red) and the LED for display (green) are lit. Light having an orange color obtained by mixing red and green is emitted from the distal end of the optical fiber for light projection. When an object is detected and a detection value of the detection stabilizes, a stable operation display lamp is lit and the LED for display (green) is extinguished. Consequently, the color of the distal end of the optical fiber for light projection changes from orange to red. A stable operation state of the photoelectric sensor can be confirmed according to the change of the color.
In alignment of the optical fiber for light projection and the optical fiber for light reception of the photoelectric sensor, by performing position adjustment while confirming the change of the color of the distal end of the optical fiber for light projection, it is possible to accurately position the optical fiber for light projection and the optical fiber for light reception to bring the photoelectric sensor into the stable operation state.
That is, the relative positioning of the optical fibers for light projection and reception is performed while confirming the operation display lamp and the stable operation display lamp of the controller. Therefore, when relative positioning work of the optical fibers for light projection and reception, that is, work of the optical axis alignment is performed in a place away from the controller, the relative positioning of the optical fibers for light projection and reception is substantially difficult. In order to solve this problem, Patent Literature 2 proposes to dispose an LED for display side by side with an LED for light projection in the controller. This makes it possible to induce, using an optical fiber optically coupled to the LED for light projection, light of the LED for display to the distal end of the optical fiber. Patent Literature 2 also proposes to light both of the LED for light projection (red) and the LED for display (green) when the object is detected and light the stable operation display lamp and extinguish the LED for display when the detection of the object stabilizes. Consequently, the color of the light emitted from the distal end of the optical fiber for light projection is changed from orange to red. According to the change of the color, it is possible to confirm the stable operation state of the photoelectric sensor even if the stable operation display lamp of the controller is not confirmed.
JP-A-2002-71553 (Patent Literature 3) discloses a separation-type photoelectric sensor including a single detection head and a controller, which are coupled by an optical fiber. The photoelectric sensor is applied to detection of a liquid spill (liquid leakage) around a tank that stores volatile liquid. The detection head has a detection surface and projects light toward the detection surface. When the detection surface is wet by a liquid spill, the light is transmitted through the detection surface and cannot be received. Therefore, a light reception amount decreases. Consequently, the liquid spill can be detected. The photoelectric sensor includes a light dividing unit that divides a part of the light upward halfway in inducing the light toward the detection surface in the detection head and includes a display unit that makes it possible to visually recognize the divided light from above the detection head.
The controller includes a red light source and a yellow light source as light projection elements. The red light source and the yellow light source are disposed to be orthogonal to each other. A half mirror is interposed between the red light source and the yellow light source. For example, the red light source is used during normal operation. When a liquid spill is detected, the red light source is switched to the yellow light source. Consequently, a red color can be visually recognized through the display unit at normal time and a yellow color can be visually recognized through the display unit during the liquid spill.
As a modification, the red and yellow light sources may be lit together at the normal time such that an orange color is visually recognized through the display unit. Further, as a modification, it is proposed to, when an abnormality such as an attachment failure of the sensor, a failure of an electric circuit, deterioration of the light receiving element, or breakage of the optical fiber is detected, change a light emission form of the light projecting element.